The invention relates to a method of providing a substrate of an optically readable information disc with an optically detectable structure, which disc can be rotated by means of a drive apparatus comprising a drive spindle and optical read means. The method employs a mould having a base provided with a mould structure which is covered with a reproduction layer, which in a deformable phase adapts itself to the mould structure, is subsequently solidified and, while attached to the substrate, is separated from the mould in such way that the structure is maintained.
Such a method is described in U.S. Pat. No. 4,312,823 (hereby incorporated by reference). The method described therein relates to the manufacture of a video long-play disc by means of a reproduction layer made of a moulding resin. The substrate is transparent and is made of a suitable synthetic material. The moulding resin may be a radiation-curable moulding resin, which is exposed through the transparent substrate. The method described therein may also be employed for the manufacture of information discs for computer memories and similar uses, on which information can be recorded by means of a suitable light source, such as a laser.
A method of optical information-recording is described in U.S. Pat. No. 4,363,116 (hereby incorporated by reference). The information disc is provided with an optically detectable servo track which extends over the entire area of that part of the information disc which is available for recording. The servo track is generally a spiral track, but in principle it may alternatively comprise a multitude of concentric sub-tracks. By means of a servo track it is possible to control the radial position of the radiation spot formed on the information layer by the radiation beam. The servo track may already contain information before the user records information on the information disc. This pre-recorded information in the servo track may comprise, for example, sector addresses in each of which the address of the associated continuous portion of the servo track is encoded in address areas. The servo track contains a multitude of sector addresses per turn of the servo track. In addition to the track number or the number of the turn of the spiral servo track, the sector addresses may, for example, also contain the number of the relevant sector in this track or in this turn. In addition, a plurality of synchronizing areas may be present at the beginning of each sector address, which when read yield a clock regeneration signal which dictates the clock frequency of the electronic clock which determines the frequency with which the information to be recorded is supplied. The layer on which recording is possible by optical means is deposited on the transparent layer provided with the servo track and may be a thin metal layer, for example one containing tellurium.
Generally, optically readable information discs should comply with very strigent requirements as regards the eccentricity of the structure, for example the servo track, relative to the axis of rotation of the information disc. In a known information disc on which recording can be made by optical means, the information disc comprises two glass substrates which are hermetically affixed to each other by means of interposed annular spacers, as in U.S. Pat. No. 4,074,282 (hereby incorporated by reference). Such a disc, which has a diameter of approximately 30 cm, contains 32,000 spiral grooves with a pitch of 1.6 micron. The eccentricity of the servo track relative to the axis of rotation of the information disc should not exceed 20 microns. It will be evident that when the servo track and the sector addresses are formed on the substrate the method of forming the structure on the substrate should comply with very strigent requirements in view of the extremely small permissible eccentricity.